The present invention relates to a device for controlling the amount of fuel supplied to an engine of an automobile etc. and, more specifically, to control of a fuel injection amount for acceleration in an engine having an electronically controlled fuel injection device or the like.
FIG. 11 shows constitution of a conventional fuel injection device for an engine disclosed, for instance, in Japanese Patent Application Examined Publication No. Sho. 62-46690. In FIG. 11, reference numeral 1 denotes an engine; 2, an intake pipe connected to the engine 1; and 3, a pressure sensor for detecting the internal pressure of the intake pipe 2. The output of the pressure sensor 3 is sent to an A/D converter 91 of a control section 9. Further, reference numeral 4 denotes a throttle valve provided in the intake pipe 2;5, a throttle sensor for detecting the opening degree of the throttle valve 4; 6, a cooling water temperature sensor for detecting the warming up state of the engine 1; and 7, an injector provided in the vicinity of each cylinder intake port of the intake pipe 2. Fuel, whose pressure has been adjusted to be constant, is supplied by pressure to the injector 7. Reference numeral 8 denotes a rotary sensor for detecting the rotation of the engine 1 in the form of pulses. The output of the rotary sensor 8 is sent to an input circuit 92 of the control section 9.
The control section 9 calculates a necessary fuel injection amount based on the outputs of the pressure sensor 3, rotary sensor 8, etc., and generates pulses in accordance with the fuel injection amount thus calculated. The injector 7 is driven based on the width of those pulses. More specifically, in the control section 9, the A/D converter 91 converts the analog signals from the pressure sensor 3, throttle sensor 5, etc. to digital data, which are sent to a microprocessor 93. The input circuit 92 level-converts the pulse signal from the rotary sensor 8. The output of the input circuit 92 is also sent to the microprocessor 93. Based on those digital data and pulse signal, the microprocessor 93 calculates the amount of fuel to be supplied to the engine 1 and generates the pulses for driving the injector 7, the width of those pulses being in accordance with the calculated fuel amount.
A control procedure of the microprocessor 93 and various data are stored in a ROM 94 in advance. A RAM 95 temporarily stores data generated in the process of calculations by the microprocessor 93. An output circuit 96 drives the injector 7 in accordance with the output of the microprocessor 93. Reference numeral 10 denotes an intake air temperature sensor for the engine 1.
Next, the operation of the above fuel injection device will be described. FIGS. 12 and 13 constitute a flowchart showing the operation of the control section 9 in FIG. 11, which flowchart is directed to the case of acceleration. A program stored in the ROM 94 is so constructed as to make the microprocessor 93 execute a timer routine 200 at predetermined intervals even when it is executing a main routine. In step 201 of the timer routine 200, the microprocessor 93 takes in an A/D-converted value THP indicating a latest throttle position from the RAM 95. In step 202, the microprocessor 93 takes in, from the RAM 95, an A/D-converted value THP' indicating a throttle position when the timer routine 200 was previously executed. In step 203, the latest value THP is stored into the RAM 95 as THP'. In step 204, a calculation of .DELTA.THP=THP-THP' is performed, where .DELTA.THP means a variation of the throttle position in a predetermined interval.
In step 206, .DELTA.THP is compared in magnitude with a judgment constant Ka for acceleration that is predetermined for the engine. If .DELTA.THP is larger than or equal to the constant Ka, the process goes to step 207, where a logical flow control flag A is set at 0. If .DELTA.THP is smaller than Ka, the logical flow control flag A is set at 1 in step 212 and a fuel injection amount correction coefficient AEWA is set at 0 in step 213. Then, the process goes to step 209, where a coefficient AEW.sub.0 is calculated by applying, to .DELTA.THP, a cooling water temperature correction, an intake air temperature correction and an atmospheric pressure correction using an atmospheric pressure sensor (not shown). More specifically, .DELTA.THP is multiplied by a correction coefficient f(THW) for a cooling water temperature, a correction coefficient f(THA) for an intake air temperature THA and a correction coefficient f(Pa) for an atmospheric pressure Pa.
Then, the process goes to step 214. If the logical flow control flag A is 0, the process goes to step 215, where AEW.sub.2 is calculated by adding AEWA that is stored in the RAM 95 to AEW.sub.0. Then, the process goes to step 216. If the logical flow control flag A is not 0 in step 214, the process directly goes to step 216, where a coefficient AEW.sub.3 is calculated by subtracting from AEW.sub.2 a subtraction constant DAEW that is predetermined in accordance with performance and characteristics of the engine. In step 218, it is judged whether AEW.sub.3 is positive or not. If it is positive, the process directly goes to step 221, where AEW.sub.3 is stored into the RAM 95 as the fuel injection amount correction coefficient AEWA for acceleration that has been calculated this time. If AEW.sub.3 is judged to be negative or zero in step 218, it is set at 0 in step 219. The execution of the timer routine 200 is finished in step 222.
On the other hand, in a fuel injection pulse width calculation routine (not shown), a basic fuel injection pulse width Tp, which is determined based on the speed of engine rotation and the internal pressure of the intake pipe 2 in accordance with the state of the logical flow control flag A, is corrected by multiplying it by (1+AEWA).
In the conventional electronically controlled fuel injection device for an engine, in calculating the fuel injection amount correction coefficient AEWA, the subtraction is performed using only the single predetermined subtraction constant DAEW, which is a fixed value. Therefore, after the fuel injection amount correction coefficient AEWA reaches a maximum, a time constant of its decrease has a decreasing pattern of first-order lag. For example, if the subtraction constant DAEW is determined for slow acceleration, after quick acceleration (during which the fuel injection amount correction coefficient AEWA is large) it will take long time for AEWA to return to zero from a time point close to the end of the acceleration. This will cause a problem that the air-fuel ratio deviates from an optimum value during a certain period after the acceleration.
Further, where the fuel evaporation is dominated by two components (low and high boiling point components) and is effectively determined by two time constants, the single subtraction constant DAEW cannot provide control of the decreasing pattern of the correction coefficient such that two time constants, i.e., two gradients are involved. This will cause a problem that the air-fuel ratio deviates from an optimum value during a certain period after acceleration, to deteriorate a driver's feeling of acceleration.